A medical image capturing apparatus such as an X-ray computed tomography (CT) apparatus, a magnetic resonance imaging (MRI) apparatus or the like is used to capture time-series images of a subject to which a contrast agent has been administered to obtain information about the blood flow dynamics of tissue by analyzing the images. This is called perfusion analysis, which uses the fact that the concentration of the contrast agent in the tissue can be obtained from the value of a pixel corresponding to the tissue in the image.
For example, deconvolution method is used as a common perfusion analysis method. In the deconvolution method, using a time density curve in an artery in the immediate vicinity of the tissue as an input function, the deconvolution (inverse convolution integration) of the time density curve of the tissue is performed to obtain the impulse response function of the tissue. Then, from the impulse response function, the blood-flow volume, average transit time, blood volume, and the like representing the blood flow dynamics of the tissue are calculated. Besides, maximum slope method may be used as another perfusion analysis method. In the maximum slope method, the blood-flow volume is calculated through the division of the maximum slope of the rising portion of transition information in the time density curve of the tissue and the maximum value of the time density curve of the artery. Note that the time density curve is a curve representing the transition of the concentration of the contrast agent (density of contrast) measured in a graph. The term “transition information” as used herein refers to information that indicates the transition of the concentration of the contrast agent.
The time-series images of the subject having been administered a contrast agent are typically captured by securing the position of the bed of the apparatus and repeatedly capturing (volume scanning) the image of a predetermined area at regular intervals. The time-series images include a plurality of still images. For example, when the medical image capturing apparatus performs volume scan 60 times at intervals of 1 second, time-series images of one minute consisting of 60 frames are obtained for the area. The medical image analyzer performs the perfusion analysis of the time-series images obtained in this way.
In capturing a large tissue such as lung, brain, liver, or the like, the image of an area including the whole tissue may not sometimes be captured. Like this, if the tissue is larger than the available imaging area, a contrast agent is administered to a part of the area of the tissue to capture time-series images of the area. This is repeated by moving the bed and securing it again. In other words, a contrast agent is administered more than once, and volume scan is performed by moving the imaging areas to photograph the entire area of the tissue in divided areas. The images may be captured such that some areas have an overlapping area.
One approach to obtain an image representing the entire tissue from images captured in divided areas is to select a vessel pixel that represents a blood vessel from pixels in each image, and associate vessel pixels thus selected as landmarks to thereby perform the registration of the images. The images after the registration form an image that represents an area including the entire tissue. In other words, registered partial images are combined together to obtain the entire area image.
The perfusion analysis requires transition information on an artery to be analyzed. However, in an image captured by capturing divided areas, an artery area may sometimes be specified in only a part of partial image areas. In this case, the perfusion analysis cannot be performed for tissue in the partial image where an artery area is not specified. This cannot be solved even by combining a plurality of partial images using a known technique. This is because the images of divided areas are captured at different times, and therefore, the transition information of an artery obtained from an artery area specified in a part of the partial images cannot be applied to those in which an artery area is not specified.
Further, in the perfusion analysis, the transition is analyzed in the pixel value representing the concentration of a contrast agent administered to capture time-series images. However, when time-series images are captured by moving the imaging area with two or more doses of a contrast agent, the contrast agent administered previous to a particular time-series image may remain in the area represented in the time-series images. In this case, transition occurs in the concentration of the contrast agent administered to photograph the time-series image and that of the contrast agent administered previous to the capturing, and thus the transition of a pixel value representing the sum of the concentrations is analyzed. This reduces the accuracy of the perfusion analysis.